1. Field of Invention
The present invention relates to an operational amplifier field, and more particularly to a rail-to-rail Miller compensation method without feed forward path.
2. Description of Related Arts
Operational amplifier is usually basic components of a signal processing and feedback system, the stability of which affects the system performance. The basic elements constituting an operational amplifier have parasitic capacitance or inductance, so the output of the operational amplifier will produce a phase shift relative to the input thereof. For an ordinary two-stage operational amplifier, its transfer function can be expressed as follows:H(s)=Av/(s+p1)*(s+p2),
wherein Av is the DC gain, p1 is the dominant pole, p2 is the secondary pole. The gain decreases by 20 dB/10 oct at every pole, and finally a 90-degree phase shift is produced at every pole. When the operational amplifier is applied to the negative feedback control system, the gain of the transfer function varies with the frequency, if before the unity gain, the phase shift caused by the pole is larger than 180-degree, then the system will oscillate, that is to say, be unstable. In general, a stable system should have a phase shift smaller than 135-degree when the gain reaches unity, it can also be said that the stable system needs at least 45-degree of phase margin.
To get a stable system, the frequency compensation is often introduced. The most common compensation method for a two-stage operational amplifier is Miller compensation, as shown in FIG. 1, wherein GM1 is the first gain stage transconductance amplifier, GM2 is the second gain stage transconductance amplifier, R1 is the first gain stage output resistor, RL is the load resistor, CL is the load capacitor, CC is the Miller capacitor. The main principle is that the first pole is separated from the second pole, even if the first pole reaches a lower frequency, the second pole will not be in the range of unity gain bandwidth. The first pole of the circuit as shown in FIG. 1 can be expressed as:p1=1/(2*pi*R1*GM2*RL*CC)p2=GM2/(2*pi*CL)
At the same time, a right half plane zero will be produced because of the feed forward effect of CC:Z=−GM2/CC 
The phase shift of 90-degree produced by the right half plane zero will affect the system stability. In order to reduce its impact, the GM2 is increased, that is to say, the output current is increased, thus leading to the increase of the power consumption. There are a lot of methods for eliminating the impact of the right half plane zero. The first method is adding a resistor connected with the CC in series. The second method can also prevent the feed forward, as shown in FIG. 2.
Referring to FIG. 2, an amplifier +A is electrically connected with the CC at X. Owing to the unidirectional transmission of the amplifier, the CC is only provided on the feedback path, and no feed forward path forms. Therefore, the feed forward path is effectively eliminated, and the right half plane zero is also eliminated.
In FIG. 2, the input end of the amplifier +A is generally electrically connected with the P-type or N-type devices. FIG. 3 shows two general structure of the amplifier +A. When the amplifier +A is shown as FIG. 3(a), to insure a proper function of amplifier +A, the voltage swing at the “OUT” node should be from VGS(M1)+VDSAT(M2) to VDD. Similarly, if the amplifier +A is shown as FIG. 3(b), to insure a proper function of amplifier +A, the voltage swing at the “OUT” node should be from GND to VDD−VGS(M4)-VDSAT(M3). As a result, the output voltage swing at the “OUT” node can not be the rail-to-rail. Once the output voltages at the “OUT” node exceeds the range of VGS(M1)+VDSAT(M2) to VDD and GND to VDD-VGS(M4)-VDSAT(M3), in FIG. 3(a) and FIG. 3(b) respectively, the amplifier +A will not work properly, thus the compensation will fail.